Conventionally, a regular inspecting process for heat exchanger tubes arranged in a steam generator of a PWR-type nuclear reactor is carried out by using an eddy current testing on an internal surface of a tube. More specifically, the process is carried out as follows: When a flaw detecting coil is inserted onto an internal surface of a tube, with an AC voltage applied thereto, an AC magnetic field is generated so that an eddy current is induced on the tube internal surface. Since this eddy current differs depending on a material for a tube, the kind, dimension and the like of a defect, it is possible to carry out a non-destructive testing on the tube by measuring a generated state of the eddy current. Actually, the measurements on the eddy current are carried out by measuring a change in an electric current flowing through the flaw detecting coil, and by analyzing the measured signal (eddy current signal), the state of the tube internal surface can be evaluated. In general, the analysis on the eddy current signal is carried out by separating the eddy current signal into an X-axis component and a Y-axis component that are different from each other by 90° in their phases, and based upon a voltage value (peak-to-peak voltage value) of an eddy current signal indicated by the root of the sum of squares of the voltage values (peak-to-peak voltage values) of the respective components and the phase of the eddy current signal indicated by tan−1 (voltage value in the Y-axis component/voltage value in the X-axis component), the determination on a defect and noise and the identification on the kind of the defect and the like are executed.
In the above-mentioned eddy current testing, an eddy current signal (flaw signal) derived from a fine defect on the internal surface of a tube tends to be mingled with electrical noise inherent to the flaw detecting coil and the flaw detecting unit and noise caused by the degree of circularity of the tube internal surface and local fine irregularities thereon (these noises are generally referred to as “base noise”. Namely, it is difficult to distinguish not only the voltage value, but also the frequency, from that of a defect, with the result that a defect that should originally be detected might be ignored. Therefore, so as not to ignore the defect that should be detected, there have been demands for reducing the voltage value of the base noise in the manufacturing steps of a tube. The manufacturer of tubes carries out an eddy current testing on the tube internal surface for each of tubes that have been manufactured, and informs the customer of a ratio of a voltage value of a flaw signal obtained by a predetermined artificial flaw to the voltage value of the base noise as an S/N ratio. Here, with respect to the factors that increase the base noise caused in association with the tube, factors such as irregularities in the outer diameter, the inner diameter and the thickness of a tube that has been subjected to a cold rolling process and a cold drawing process, a dimensional change due to the straightener, and a change in the thickness due to a polishing process on the tube surface; therefore, by taking these noise-increasing factors into consideration, the manufacturer produces tubes in a manner so as to satisfy the required specifications of the customer.
Specifically, the conventional measurements on the S/N ratio are carried out in the following manner. First, the measurement on the voltage value of the base noise has been carried out by allowing the operator to confirm a voltage value of an eddy current signal (eddy current signal waveform) obtained by carrying out an eddy current testing on the internal surface of a tube in an axis direction, through manual operations. More specifically, the operator visually reads the voltage value of an eddy current signal waveform outputted from a flaw detecting unit over the entire length of the tube, and the maximum value thereof is defined as a voltage value of the base noise. Alternatively, the operator visually reads the respective voltage values of signal waveforms in the X-axis component and Y-axis component outputted from a flaw detecting unit over the entire length of the tube, and the root of the sum of squares of the maximum voltage values of the respective components is defined as the voltage value of the base noise. Then, a voltage value of an eddy current signal or the root of the sum of squares of the respective voltage values in the X-axis component and Y-axis component, obtained by a predetermined artificial flaw is defined as the voltage value of a flaw signal, and the ratio of the voltage value of this flaw signal to the voltage value of the base noise is calculated as the S/N ratio.
However, in the above-mentioned conventional S/N ratio measuring method, the operator has to confirm waveforms of eddy current signals (or the X-axis component and Y-axis component thereof) over the entire length of a long tube, outputted from a flaw detecting unit, and by visually reading the maximum amplitude of each of these signal waveforms, the operator needs to determine the voltage value of the base noise. For this reason, a problem arises in which the working efficiency deteriorates (for example, it takes about one minute to measure the S/N ratio of one tube of about 20 m in length). Moreover, in some cases, it is difficult to determine whether the measured base noise is noise derived from the shape or the like of the tube, or electrical noise inherent to the flaw detecting unit or the like, and in such a case, an eddy current testing needs to be again carried out for confirmation so as to distinguish the cause of the noise generation. For this reason, more time is required, and another problem arises in which the higher degree of skill of the operator relating to the determination is required.
Here, in order to reduce the base noise, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 8-211026) has proposed an eddy current sensor probe that is designed so that prior to giving a detection signal detected by a flaw detecting coil to a signal cable, it is amplified by an amplifier. Moreover, Patent Document 2 (Japanese Utility Model Application Laid-Open No. 5-28962) has proposed a detecting probe for a fine tube in which, by attaching a probe head formed by a cylinder member made of synthetic resin to the front end of a flaw detecting probe so as to prevent rattling, noise generation due to a change in the inner diameter such as a tube expanding process or a tube constricting process can be prevented.